A conventional noise shaping circuit is basically composed of an integration circuit and a quantizer.
In a first conventional noise shaping circuit, the integration circuit comprises serially connected first to third adders, first to third delay units each feeding an output of a corresponding one of the first to third adders back to the corresponding adder, and an additional delay unit feeding an output of the quantizer back to the first adder. Thus, the integration circuit having three integration steps is provided. In such a noise shaping circuit, the noise shaping property is improved to be proportional to the number of the integration steps.
However, the first conventional noise shaping circuit has a disadvantage in that oscillation occurs in the integration circuit, as the number of the integration steps is increased. In order to overcome this disadvantage, the precision of quantized bits is increased in the quantizer. However, this results in a complicated structure of an output circuit and the necessity of a higher frequency clock.
In a second conventional noise shaping circuit, the integration circuit comprises first to third sets of first to third adders and first to third delay units, coefficient multipliers for multiplying outputs of the second and third adders by coefficients, a limiter provided between the third adder and a corresponding one of the coefficient multiplier, and an additional delay unit feeding an output of the quantizer back to the first adder.
In a second conventional noise shaping circuit, the coefficient multipliers are provided to decrease the influence of an oscillation state, and the limiter is also provided to suppress oscillation occurring at the third integration step.
However, the second conventional noise shaping circuit has a disadvantage in that the integration circuit works only as having two of the first and second integration steps, because the limiter suppresses even the integration operation of the third integration step.
In a third conventional noise shaping circuit, the integration circuit comprises first to third coefficient multipliers, and two delay units, wherein an output of the quantizer is fed through a first path of the two delay units and the first coefficient multiplier, a second path of one of the two delay units and the second coefficient multiplier, and a third path of only the third coefficient multiplier back to the adder. In accordance with the structure, noise is decreased at an audio frequency band.
However, the third conventional noise shaping circuit has a disadvantage in that oscillation can not be avoided, although the noise shaping property is improved at the audio frequency band.